Supercritical fluids or solutions exist when the temperature and pressure of a solution are above its critical temperature and pressure. In this state, there is no differentiation between the liquid and gas phases and the fluid is referred to as a dense gas in which the saturated vapor and saturated liquid states are identical. Near supercritical fluids or solutions exist when the reduced temperature and pressure of a solution are both greater than 80% of their critical point but the solution is not yet in the supercritical phase. Due to their high density, supercritical and near supercritical fluids possess superior solvating properties.
Supercritical fluids have been used in thin film processing as developer reagents or extraction solvents. Morita et al. (U.S. Pat. Nos. 5,185,296 and 5,304,515) describe a method in which supercritical fluids are used to remove unwanted organic solvents and impurities from thin films deposited on substrates. Allen et al. (U.S. Pat. No. 5,665,527) describe a high resolution lithographic method in which a supercritical fluid is used to selectively dissolve a soluble unexposed portion of polymeric material from a substrate, thereby forming a patterned image. In recognition of the superior solvating properties of supercritical fluids, Steckle et al. (U.S. Pat. No. 5,710,187) describe a method for removing impurities from highly cross-linked nanoporous organic polymers.
Methods for depositing thin films using supercritical fluids also have been reported. Murthy et al. (U.S. Pat. No. 4,737,384) describe a method for depositing metals and polymers onto substrates using supercritical fluids as the solvent medium. Sievers et al. (U.S. Pat. No. 4,970,093) describe a chemical vapor deposition method (CVD), in which a supercritical fluid is used to dissolve and deliver a precursor in aerosol form to a conventional CVD reactor. Watkins et al. (U.S. Pat. No. 5,789,027) describe a method termed Chemical Fluid Deposition (CFD) for depositing a material onto a substrate surface. In this method a supercritical fluid is used to dissolve a precursor of the material to be deposited. Once dissolved, a reaction reagent is introduced that initiates a chemical reaction involving the precursor, thereby depositing the material onto the substrate.
Although the above mentioned methods take advantage of supercritical fluids as mediums for reagent transport, reaction, and removal of impurities, what is lacking in the art are more reliable and practical apparatus and methods of using them. Conventional methods and apparatus that use supercritical fluids for depositing films on substrates involve batch type processes, where a substrate and a precursor or reactant are placed in a reactor. The reactor is then charged with a supercritical fluid. In this way, the precursor or reactant is dissolved and the substrate exposed to the supercritical solution. Once the deposition (or other) reaction is complete the reactor is vented and the substrate removed. Such methods and apparatus are especially problematic when the precursor or reactant is a solid. Often times it is either difficult to dissolve the solid properly within the reactor, or the deposition is not uniform due to obligatory dissolution of the precursor in the presence of the substrate. This dissolution often involves heating the precursor which can cause side reactions, which may form unwanted impurities on the substrate or in the deposited layer thereon.
What is therefore needed are improved apparatus and methods for introduction of solid precursors and reactants into a supercritical fluid reactor. In particular, what is needed are apparatus that deliver preformed solutions of solid precursors and reactants to supercritical fluid reactors.